Amorphous In-Ga-Zn-O (a-IGZO) thin films (similar to 200 nm thickness) were deposited by radio-frequency (RF) magnetron sputtering on silicon and glass substrates at various working pressures (0.67-2.67 Pa) and a fixed oxygen-to-argon gas-flow ratio (O-2/Ar = 5%). The transparency of all of the films was more than 85% in the visible range. With increased working pressure, the surface morphology of the films, as observed under atomic force microscopy (AFM), became rough; the optical band gap, estimated by Tauc plot, increased, and the mobility and carrier concentrations, according to Hall measurement, decreased and increased, respectively. The resistivity of the films initially decreased (up to 2.00 Pa working pressure) and then increased (at 2.67 Pa). It is suggested that the electrical property changes were affected by the role of the oxygen vacancies, whether as effective donors or as scattering centers. (C) 2012 Elsevier Ltd. All rights reserved.